The present invention relates to a drill bit for use in combination with a drill string for drilling a borehole into an earth formation. In the industry of drilling wellbores it is conventional practice to drill boreholes at stepwise decreasing diameters in downward direction. This is normally a consequence of the casing or liner program whereby casing or liner sections of stepwise decreasing diameters are installed in the wellbore. In such arrangement each next lower casing or liner section extends through a previously installed section. As a result the upper part of the wellbore has to be drilled at a significantly larger diameter than the lower parts of the wellbore, thereby increasing the costs of drilling.
It is desirable to reduce the costs of drilling and to provide a drill bit which obviates the need for a stepwise decreasing borehole diameter.
Furthermore, it is desirable to provide a drill bit which is capable of underreaming an existing borehole.
In accordance with the invention there is provided a drill bit suitable to be connected to a drill string for drilling a borehole into an earth formation, the drill string having a central longitudinal axis, the drill bit comprising
a bit body connectable to the lower end of the drill string; and
a cutting member provided with cutting means at the outer surface thereof, the cutting member being rotatable relative to the bit body about a longitudinal axis of rotation extending eccentrically relative to the central longitudinal axis of the drill string, between a first rotational position and a second rotational position, whereby in said second rotational position the cutting means is arranged at a larger radial distance from the central longitudinal axis of the drill string than in said first rotational position.
It is thereby achieved that by rotating the cutting member from the first rotational position to the second rotational position, the cutting diameter of the drill bit increases. In an attractive application the drill string with the drill bit connected thereto, is lowered through a casing of an upper borehole portion to an uncased lower borehole portion while the cutting member is in the first rotational position. Thereafter a further borehole section is drilled with the cutting member in the second rotational position, whereby the diameter of the newly drilled portion is larger than the internal diameter of the casing through which the drill bit was lowered. A new casing section is then lowered through the upper casing into the lower borehole portion, which new casing section then is expanded against the borehole wall. Thus, the borehole diameter is allowed to remain unchanged in downward direction.
Preferably the direction of rotation of the cutting member from the first rotational position to the second rotational position is opposite the direction of rotation of the drill string during normal drilling of the borehole using the drill string and the drill bit. When the drill string starts rotating with the cutting member in the first rotational position, the cutting member is subjected to a drag force due to contact with the borehole wall, which drag force induces the cutting member to rotate in opposite direction relative to the bit body. As a result the cutting member rotates from the first rotational position to the second rotational position thereof, whereby the cutting diameter of the drill bit increases.
Advantageously the cutting member is movable in axially direction relative to the bit body between a primary axial position and a secondary axial position, wherein in the primary axial position the cutting member is located closer to the lower end of the drill string than in the secondary axial position, and wherein the bit body and the cutting member are provided with co-operating guide means for guiding the cutting member from the first rotational position to the second rotational position upon movement of the cutting member from the primary axial position to the secondary axial position. Thus, the cutting member can be moved from the first to the second rotational position by, for example, applying weight to the drill string resulting in lowering of the string whereby the drag forces from the borehole wall induce the cutting member to move from the primary to the secondary axial position. Alternatively, or in addition, such axial movement can be achieved by operating a hydraulic actuator incorporated in the drill string.